1. Field of the Invention
The present invention relates to correction of brightness irregularities in a display device.
2. Description of the Related Art
FIG. 1 shows the structure of a circuit for one pixel section (pixel circuit) of a basic active organic EL display device, and FIG. 2 shows the structure and input signals of a display panel.
A data signal is written to a storage capacitor C by setting a gate line (Gate), that extends in the horizontal direction), to a high level to turn an n-channel selection TFT 2 on, and in this state placing a data signal (image data) having a voltage corresponding to a display brightness on a data line (Data) that extends in the vertical direction. In this way, a gate of a p-channel drive TFT 1 is set to a voltage corresponding to the data signal, drive current corresponding to the data signal is supplied to an organic EL element, and the organic EL element emits light.
In FIG. 2, pixel data, a horizontal sync signal (HD), a pixel clock and other drive signals are supplied to a source driver. The pixel data signal is sent to the source driver in synchronism with the pixel clock, held in an internal latch circuit once a single horizontal line of pixels have been acquired, and subjected to D/A conversion all at once to supply to a data line (Data) of a corresponding row. Also, the horizontal sync signal (HD), other drive signals and a vertical sync signal (VD) are supplied to a gate driver. The gate driver performs control to sequentially turn on gate lines (Gate) arranged horizontally along each line, so that image data is supplied to pixels of the corresponding line. The pixel circuit of FIG. 1 is provided in the pixel sections that are arranged in a matrix shape. Also, a power supply line PVDD is arranged in the vertical direction along a pixel row, and CV is connected to a power supply CV with cathodes of the organic EL element provided common to all pixels.
As a result of this type of structure, data is sequentially written to each pixel in horizontal line units, and display is carried out at each pixel in accordance with the written data, to perform image display as a panel.
Here the amount of light emission and current of the organic EL element are in a substantially proportional relationship. Normally, a voltage (Vth) is supplied across the gate of the drive TFT and PVdd such that a drain current approaching that for a black level of the pixel starts to flow. Also, the amplitude of the image signal is an amplitude so as to give a prescribed brightness close to a white level.
FIG. 3 shows a relationship for current “CV current” (corresponding to brightness) flowing in the organic EL element with respect to input signal voltage (voltage of the data line Data) of the drive TFT. It is possible to carry out appropriate gradation control for the organic EL element by determining the data signal so that Vb is supplied as the black level voltage and Vw is supplied as the white level voltage.
Specifically, the brightness when the pixel is driven at a particular signal voltage differs depending on the threshold voltage (Vth) of the drive TFT, and an input voltage close to PVdd (power supply voltage)−Vth (threshold voltage) corresponds to a signal voltage when displaying black. Also, the slope (μ) of the V-I curve of a TFT varies in a similar manner, and in this case, as shown in FIG. 4, an input amplitude (Vp−p) for outputting the same brightness is also different.
If there are variations in Vth and μ of the TFT inside the panel, there will usually be inconsistencies in brightness. With the objective of correcting these brightness inconsistencies, panel current flowing when lighting up each pixel at a number of signal levels is measured, to obtain a V-I curve for individual TFTs.
A correction data calculation method is shown in FIG. 5. First, a V-I characteristic curve for standard pixels of the panel is obtained by measuring a voltage to current characteristic for a number of pixels. It is assumed that this curve is represented by an equation such as Id=f(a(Vgs−b)), and a function f(x) is determined. A characteristic for all pixels of the panel is represented by this f(x), and if it is assumed that variation in characteristic is due to difference between a coefficient a and a coefficient b, a and b for each pixel can be obtained by measuring pixel current corresponding to two or more input voltage levels.
When the V-I characteristic of a pixel p is represented by Id=f(a′(Vgs−b′)), correction is carried out by first obtaining offset=k(b′−ab/a′) and gain=a/a′ using a and b of previously obtained average pixels, with k as a D/A conversion coefficient, and the image data is then multiplied by the obtained gain and added to offset.
In the case of carrying out this type of processing, as shown in FIG. 6, first γ correction is carried out in a γ look up table (LUT) in order to compare relationships between pixel data and pixel current for image data (R signal, G signal and B signal), and image data that has been γ corrected is obtained. Next, image data after γ correction is multiplied bγ correction gain in a correction calculation section 12, and irregularities are corrected by adding the correction offset.
Image data (R, G, B) for which irregularity has been corrected is supplied to the display panel 14, where it is displayed. Here, correction gain and correction offset for every pixel is stored in a memory section such as RAM, read out in synchronism with image data, and used in correction of the image data.